So I have this circa 2007 Calfee frame that is supposed to be "Gates compatible" according to the shop we got it from. Apparently all that means it has the requisite spacing between the two bottom brackets because it seems with a normal install configuration there is not enough clearance between the rear ring and the chainstay. The cranks are centered (no shift to one side or the other) and this frame does not have the increased ring clearance on the newer Calfee frames (that looks like a big dent). I am using the new Shimano Ultegra Hollowtech II tandem triple cranks (FC-R601/FC-R603) with a standard 68mm BB.

One poor solution is to try adding a bunch of spacers to the spider. Not my first choice.

What I have done (and need to validate) is to flip the rings and bolts around so the ring flange w/bolt heads is on the inside of the crank spider and this works perfectly for the clearance issue. The Ultegra spider seems perfectly capable of handling a ring on either the inside or outside. As a bonus, there is no way the belt will ever rub the chainstay as some people have experienced. So the question here is if mounting the rings this way would cause any known problems?

The second question is about the belt and tracking. I believe we have the rings in perfect alignment as measured by equal offset from the bottom tube. I have not yet tightened the eccentric, but slipped the belt on with some slack remaining. With this loose mock-up, the belt simply slips off the side without any restraint. The question is, at what point does the belt teeth sink in past the lower of the ring edges so that it will stay in the tracks? Does full engagement only occur with the full tension applied?

Not 100% sure about the tandem belt drive, but on the singles they have the chainring (beltring?) with the edge lip on the outside (opposite of your current setup) and the rear cog with the edge lip on the inside (similar to your setup). Until you have the tension set, the belt won't engage the chainring correctly.

In looking at your setup, I'd speculate that your belt is too large. Not sure you have enough movement in your bottom bracket to tighten that properly. I'd check to make sure you can get that tight enough. If your tension can get tight enough, I don't think you'll have any problems with the direction of your chainrings.

My 2-cents.

I am concerned about the BB/eccentric ability to provide enough tension too. The shop we bought this frame from insisted it is "Gates compatible" and that Calfee has been building them to a standard bottom tube length required for the Gates belt drive. Since the eccentric is already currently in the 4.5 o'clock position and we are just getting 28.5" at that position, I agree with your observation. I think our bottom tube center-center of the BBs measures 72cm, which logically matches the stoker top tube length given our frame has identical 74 degree seat tubes for the stoker & captain. Based on that, I think we are maybe a 1/2 cm short of the optimal bottom tube length. But doesn't Calfee build most all their current frames with a 72cm stoker top tube and same seat angles?

I'm picking up a ball-end hex tool tomorrow to fit into the eccentric bolt heads with the BBs installed - the design of this particular eccentric makes for a very limited tool fit.

On the belt engagement, the belt needs to be fully tensioned to get proper belt engagement with the pulley/sprockets and, even then, the static tension is no where near what the belt experiences along the top run in use. More on belt tension in my 24 Nov 2010 blog entry on the subject. As someone else noted, there sure seems to be a lot of slack in that belt. Most of the belts I've worked on are very tight even with the eccentric in the most re-ward position.

As for alignment, you'll only know if you've got the alignment dialed-in when you tension the belt and can successfully rotate and counter-rotate the cranks in the work stand without the belt walking off the pulley/sprockets.

Finally, as for having the rings reversed with the flange on the inboard side... assuming the belt can be used on your tandem (re: length issue) both of you should keep a close eye on the belt during your initial rides. If either of you see the belt walking off your respective pulley/sprocket, you'll have your answer. In theory -- and with both the sprockets aligned -- the belt 'should' track quite well even without any flange(s). Taking it a step further, if your cranks, bottom brackets and boom tube have zero deflection under load.. there shouldn't be any issues. Therefore, the question becomes, how much if any elasticity is there in the bottom end of your Calfee as its configured? I would expect it to be fairly low (a lot lower than steel, Ti or aluminum), but I'm not sure if it's zero or just how much tolerance the belt drive will have for any rider-induced sync drive-side deflection.

A friend of ours who did a lot of independent work on tandem sync-drive's using the pre-Carbon Drive synchronous belts from Gates and who beta tested various configurations on their Seven 007 titanium tandem theorized that early testing on steel and alloy tandem frames that are prone to some bottom end sync-drive side deflection would have revealed the need for at least one of the flanges having an outboard flange to keep the belt from walking under heavy pedal loading, but never bothered to test any sprockets that didn't have flanges on both the inboard and outboard edges of his custom machined pulley/sprockets.

This install is using the standard 69T rings. Apparently Santana uses 74T rings that are also 8mm pitch (Gates part number 08745AA10), but I'm not finding any specifications for those as far as ring diameter or spacing requirements. I wonder how much larger they are and if that might be a solution here, if we can't get the belt tight enough... hmmm.

As for flanges on one side or the other I guess that is a non-issue. It would seem the only ring orientation issue as far as reversing them (placing on the inside) would be if the teeth were somehow machined to only work in a single direction of rotation.

I believe Santana's current boom tubes use 27.75" (70.5 cm) C-T-C spacing.

As I said before, until you tried the bike with the pulley/sprockets reversed you won't know for sure if the belt will walk under load. Ref. frame flex, while Gates doesn't discuss the tandem sync drive and single side flanges, they do acknowledge the issue that frame flex can have on their system in this FAQ:

This is an issue with the new Ultegra tandem cranks - the timing-side chainline is a couple mm less than on the FSA cranks (51mm instead of 53mm IIRC). I've been running a different set of captain's cranks (the FSA cranks are too long for me) with some small spacers (2mm I believe) behind the belt ring for a while now without any issue, and I'm not worried about it, so that is what I would do here.

As for flipping the belt ring over, then it should work just fine. In the normal orientation, the lip of the ring does provide some protection to stop things getting caught between the belt and ring, and you will not have that feature, but neither do the center track versions of the belt drive, so it's obviously not essential. On our setup, we get very odd noises and the belt creeps around a bit if we install it reversed to normal; other people don't seem to have that problem, but do try flipping the belt if it is making noise or not staying aligned.

We had 71t pulleys on our CoMotion. These would reduce the crank centre distance by 8mm compared to the 69t.

Interesting. I think 71T would be perfect here, however Gates does not list that ring size (https://www.carbondrivesystems.com/cd_specs.php?lang=us). I'll have to call around.

Have you talked to Michael at Calfee? He has been very helpful

Thanks for the nudge. I called Mike and he is investigating the options including availability of the 71T rings and a Bushnell "Light" eccentric he thinks might fit in my frame without mod.

Apparently Calfee had a lot of changes in the 2007-2009 timeframe centering around the Gates belt issues including spacing and alignment. My frame is not classified as being Gates compatible, contrary to the seller's claims.

I will continue to work on the Gates issue as info and parts arrive, but meanwhile on with the 'ol rings & chains.

FWIW, my Calfee was built with 71t rings.

We have the 71T belt rings. They are a little too big on a stock Co-Motion frame that they were basically designed for - with the eccentric in the loosest position, the belt only JUST slides on, with a little help of turning the cranks that goes against Gates' instructions. The belt is then already at the recommended tightness, and so the eccentric stays where it is. After less than one year, Gates realized that this was a problem and so switched to the slightly smaller 69T rings.

I sent out (2008?) frame back to Calfee to get chainstay redone for Gates ring clearance, and the following year had the eccentric redone so it could take the Bushnell. Calfee swapped my 71t rings for 69s, I think they said that would be better.

I have a question since we are talking about Gates rings.

Is it possible to run the 60t Gates rings on a tandem if the frame has a longer boom tube length?
By going from a 69t to a 60t what would be the difference in length from C to C?
Is there any disadvantage to running the smaller rings?

Thank you for an answer to my question.

Late response to the above question, but since it remained unanswered let me try to provide a SWAG.

Reference the following for ring and belt sizes available:
https://www.carbondrivesystems.com/do...Aug2012_sm.pdf

The tandem rings & 2000mm belt are 8mm pitch (tooth/belt nub size), whereas the single bike pitch is 11mm. Simply divide the belt length by the number of teeth (nubs) and you get the pitch. So you cannot mix and match rings and belts from single and tandem products.

My new question:

I would like to hear from some math whizes if it is possible to use a combination of the smaller single bike rings (5 arm, 130mm BCD) and single belts listed in the pdf above, to achieve the same "chainstay" (bottom tube) length as the gear designated for tandems. I am looking for an equivalent setup for a 71.5-72cm bottom tube length. I was unable to use the Gates calculator (https://www.carbondrivesystems.com/fo...rs.php?lang=us) to find the answer.

Any takers?

Definitely not a math whiz, but I'll take a swing --

Smallest "sprocket" (chainring) I see on the Gates PDF in 5-arm/130 is a 46 tooth. The longest belt is 125 teeth. Since we're using two chainrings and the belt covers half of each, that's 46 teeth that are in contact with the chainrings at any moment. 125-46=79 teeth left to span the distance between the chainrings. Divide by two since the belt is a circle -- 79/2= 39.5 teeth. 39.5 *11mm per tooth = 434.5mm separation between the contact points on the chainrings, which should be equal to the separation between the bottom bracket axles. Convert to inches 434.5/25.4 = ~17.11"

So.... not on any normal tandem.

Thank you. I like that your calculation uses logic rather than trig functions. Simple

As a sanity check, I ran the same process for the standard tandem 69T rings & 250T/2000mm belt. Result came out to the correct 72.4cm tube length required.

Future spec:
The Gates rep I spoke with the other day indicated they are considering changing the tandem pitch to 11mm (same as singles) in the future, which would provide for many more ring options. In that case, the 2000mm belt would yield near 182 teeth (181.8181 to be more precise). The matching 11mm pitch rings for the standard bottom tube length would be 50T to acheive 72.6cm which is probably close enough to the current spec. The Santana spec would likely use the 55T/11mm rings (yields 69.85cm). So it unfortunately looks like there will still be no solution for the case (mine for example) where the current 71T rings are needed.

BTW, the only reasoning they had for no longer producing the 71T was that they were building only for current framebuilder/OEM "standard" sizes, regardless of losing the business of frame owners with all the previous builds. I doubt there is sufficient market to justify anyone licensing the ring spec from Gates and produce 3rd party versions / more sizes.

I think that's shortsighted on Gates' part. My gut tells me tandems have a longer "lifespan" than single bikes because they cost so much to begin with. So there are tons of older tandems out there that could benefit from belt drive if it were available. They are artificially limiting their market.

If you really wanted a belt drive you could get some 71t pulleys made. It wouldn't be cheap, but certainly possible. Don't think there would be any licensing issues. A lot of Gates belts in other applications are used on pulleys made by other people.